A narrowed band of a communication path has been used by a wireless appliance from the viewpoint of improving the utilization efficiency of a radio frequency band. For example, some wireless appliances are obliged to provide one communication path per 6.25 kHz from the year of 2005 by part 90 of CFR (Code of Federal Regulation) 47 defined by the U.S. FCC (Federal Communications Commision). Those wireless appliances that cannot meet this condition cannot be granted by type approval on and after the year of 2005 and cannot be sold in the U.S.
For the wireless appliance that operates in the bandwidth of 6.25 kHz, a modulation spectrum in the transmitter is required to be in a range of transmission spectrum mask called “Mask E” as shown in FIG. 7. The restrictions by the transmission spectrum defined by the “Mask E” is such that a transmission spectrum in the frequency departing from a carrier (center frequency) by ±3 kHz is less than −30 dB, a transmission power in the frequency departing from a carrier by ±4.6 kHz for a wireless appliance with the transmission output power more than 10 W shall be less than −65 dB, and a transmission spectrum shall be below a line passing −30 dB in ±3 kHz and −55 dB in ±4.5 kHz in the region between ±3 kHz and ±4.6 kHz from a carrier.
Conventionally, the analog FM scheme for transmitting a carrier subjected to direct FM (Frequency Modulation) with an audio signal is adapted in a wireless appliance that operates in the bandwidth of 12.5 kHz or in the bandwidth of 25 kHz. The frequency modulation ratio, however, needs to be significantly small in order to match a transmission spectrum mask defined by “Mask E” as shown in FIG. 7. As such, the conventionally used analog FM scheme significantly degrades the S/N ratio so that a performance sufficient to bear practical use cannot be obtained. Thus, it is considered that a four-level FSK signal, which is resulted from all of audio signals and control signals converted into a digital signal and a carrier being subject to FSK (Frequency Shift Keying) modulation, is transmitted to match the transmission spectrum mask so that sufficient performance is obtained.
As a wireless communication system operated according to the FCC rules and regulations, a system for providing a communication service for various wireless appliances by a trunk scheme in which a plurality of channels such as audio channels and control channels are arranged is known (for example, Japanese Patent No. 2724917).
The wireless communication system in the trunk scheme can improve the efficiency of utilizing a frequency as compared to a wireless communication system or the like, in which each wireless appliance occupies an individual wireless channel, as a plurality of wireless appliances that make mobile wireless stations share a predetermined number of wireless channels. Then the wireless communication system that transmits and receives the four-level FSK signal can also adopt the trunk scheme so that it is expected to improve the efficiency of utilizing a frequency as compared with a conventional system together with rules and regulations according to the above-mentioned FCC.
FIG. 8 is a diagram showing an example of spectral distribution in a four-level FSK signal that is matched with a transmission spectrum mask of “Mask E”. As shown in FIG. 8, the four-level FSK signal can be matched with a transmission spectrum mask of “Mask E” in a predetermined experimental environment. Referring to FIG. 8, the power level in the adjacent channels detuning by 6.25 kHz is a level at the noise floor. From this viewpoint, an electric power leaked to the adjacent channels seems to be sufficiently restrained.
As the FSK modulation, however, has a nonlinear characteristic in contrast to the AM (Amplitude Modulation), it has a feature in which a transmission spectrum is apt to spread as compared with amplitude modulation or the like. If a bandwidth of a filter (BPF; Band Pass Filter) included in the receiver in the actual use environment is ±2 kHz, a spectrum component existent over the range of Δ4.25 kHz to the range of Δ8.25 kHz centering on the transmission frequency (carrier frequency) can give an influence in the form of an electric power leaked to the adjacent channels. Also in FIG. 8, existence of a frequency component in the four-level FSK signal over the range of Δ4.25 kHz centering on the transmission frequency is recognized. That increases, therefore, the level of interference to the adjacent channels in a wireless appliance that operates in the bandwidth of 6.25 kHz, as compared to the level of interference to the adjacent channels in the wireless appliance that operates in the conventional bandwidth of 12.5 kHz.
In the wireless communication system in the trunk scheme, while each of a plurality of wireless channels is set to any of a control channel and a call channel, some signals may be always transmitted from the base station over the control channel in order to control a line. At the side of the wireless appliance that makes a mobile wireless station, place registration or call control is performed as a signal always transmitted from a base station over a control channel is received. Here, the call channel is activated and transmission and reception are performed only when a line control apparatus placed in the base station performs allocation to the mobile wireless station. Thus, no signal is transmitted when the call channel is in the unused idle state. As such, in the radio frequency wave band adjacent to the call channel, the degree of influence from the signal transmitted and received over the call channel is relatively small.
On the contrary, as a signal is always transmitted over the control channel, it is always possible to have interference during a period in which a signal is transmitted over the control channel when the frequency band adjacent to the control channel within the same wireless communication system is used as a call channel or when the frequency band adjacent to the control channel in another wireless communication system is used. As such, a problem occurs in that the degree of interference increases by a signal transmitted and received over the control channel in the radio frequency band adjacent to the control channel.
The present invention is adapted in view of the above-mentioned situations and intends to provide a wireless communication control apparatus and a wireless communication method that can reduce influence to the adjacent channel as matched to the communication path that is made into the narrow-band.